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Predicted and described by Mendeleev as ekaaluminum, and discoveredspectroscopically by Lecoq de Boisbaudran in 1875, who in the same year obtained the free metal by electrolysis of a solution of the hydroxide in KOH.Gallium is often found as a trace element in diaspore, sphalerite, germanite, bauxite, and coal. Some flue dusts from burning coal have been shownto contain as much as 1.5% gallium. It is the only metal, except for mercury, cesium, and rubidium, which can be liquid near room temperatures; thismakes possible its use in high-temperature thermometers. It has one of the longest liquid ranges of any metal and has a low vapor pressure even at hightemperatures. There is a strong tendency for gallium to supercool below its freezing point. Therefore, seeding may be necessary to initiate solidification.Ultra-pure gallium has a beautiful, silvery appearance, and the solid metal exhibits a conchoidal fracture similar to glass. The metal expands 3.1% onsolidifying; therefore, it should not be stored in glass or metal containers, as they may break as the metal solidifies. Gallium wets glass or porcelain,and forms a brilliant mirror when it is painted on glass. It is widely used in doping semiconductors and producing solid-state devices such as transistors.High-purity gallium is attacked only slowly by mineral acids. Magnesium gallate containing divalent impurities such as Mn+2 is finding use incommercial ultraviolet activated powder phosphors. Gallium arsenide is capable of converting electricity directly into coherent light. Gallium readilyalloys with most metals, and has been used as a component in low-melting alloys. Its toxicity appears to be of a low order, but should be handled withcare until more data are forthcoming. Natural gallium contains two stable isotopes. Twenty three other isotopes, one of which is an isomer, are known.The metal can be supplied in ultrapure form (99.99999+%). The cost is about $4/g. 1

•&nbsp"It is used in transistors and high temperature thermometers. Gallium-67 was one of the first artificially produced isotopes to be used in medicine. It concentrates in inflamed areas and in certain melanomas." 2
•&nbsp"Perfectly conducting materials al!ow electrons to move freely among the atoms of the material. At normal temperatures, there are no perfect conductors, but metals like silver and copper are very good.

Semiconductors, on the other hand, rely on the jumping of electrons between accessible energy levels for current to flow in the material. In a solid, these levels are more numerous than in a gas of the same species and form broad bands of available energies, allowing for large congregations of electrons. These electrons undergo random thermal motion, but cannot jump to the next energy band if the gap between bands is much greater than the average thermal energy. So current flow in semiconductors is strongly dependent on the ambient temperature. Electrons can also be made to jump between bands if they absorb energy in any other form, such as electromagnetic energy. An applied electric field or light absorption can make current flow in semiconductors. It is this property that makes them useful in circuits - small amounts of current can be made to flow at prescribed moments. This is an important property in transistors, optoelectronics, and solar cells.

The energy bandwidths and gaps are also dependent on the exact nature of the material. Silicon and germanium, for example, are good semiconductors, while carbon is not, though all three have similar electronic structure. (Note that they inhabit the same column in the periodic table.) The difference is that, in solid form, the gaps between the two highest energy levels in silicon and germanium are much smaller than in carbon, so electrons are able to transition between them with ease. The addition of tiny bits of impurities can also have an impressive effect on the bandwidths and gaps.

A small amount of arsenic added to crystal gallium has resulted in a material (gallium arsenide or GaAs) that is particularly well adapted to many important semiconductor applications. Compared to the more common silicon semiconductors, GaAs displays relatively low sensitivity to heat, higher electron mobility, and the ability to withstand higher applied voltages. This means that GaAs transistors can be operated at higher frequencies and higher power with less associated noise.The nature of the band gap in GaAs also means that it is better at emitting light than silicon semiconductors, making it a material of choice for light-emitting diodes (LEDs). Though gallium is much more expensive than silicon, its high absorptivity allows for very thin films to be used as efficient collectors for solar cells. It has been used for this purpose in several Mars rovers.

Challenges for the GaAs applications include its expense. Researchers are working on finding appropriate inexpensive substrates for growth of the GaAs wafers or films. In addition, arsenic is toxic and carcinogenic, so great care must be taken toward worker safety in the wafer-polishing process." 3
•&nbsp"Almost all of the world's gallium supply is used in the electronics industry along with other applications and may be found in radios, televisions, and calculators. Gallium arsenide and gallium phosphide are used in a variety of semiconductor devices, including diodes and laser diodes, and may be found in computers and cell phones. Gallium arsenide is also used in solar batteries. Small amounts of gallium are found
in thermometers, solders, arc lamps, batteries, and catalysts." 4

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